The invention relates to a mirror for the EUV wavelength range. Furthermore, the invention relates to a projection objective for microlithography comprising such a mirror. Moreover, the invention relates to a projection exposure apparatus for microlithography comprising such a projection objective.
Projection exposure apparatuses for microlithography for the EUV wavelength range have to rely on the fact that the mirrors utilized for the exposure or for the imaging of a mask into an image plane have a high reflectivity since, firstly, the product of the reflectivity values of the individual mirrors determines the total transmission of the projection exposure apparatus and since, secondly, EUV light sources are limited in terms of their light power.
Mirrors for the EUV wavelength range around 13 nm having high reflectivity values are known for example from U.S. Pat. No. 7,474,733 B1. The mirrors shown in U.S. Pat. No. 7,474,733 B1 have high theoretical reflectivity values for multiple coatings, wherein the multiple coatings for the highest reflectivity values comprise more than 30 identical periods of silicon and ruthenium layers (Ru/Si-multilayer).
What is disadvantageous about these mirrors, however, is that said theoretical reflectivity values cannot be achieved in practice since alternating layers composed of silicon and ruthenium tend toward interdiffusion, whereby a loss of contrast occurs at the transition of the layers, which in turn leads to a reduction of the reflectivity.
Interdiffusion barriers that can prevent such mixing of different materials are known in connection with different chemical elements than silicon and ruthenium, from U.S. Pat. No. 6,396,900 B1. In this case, U.S. Pat. No. 6,396,900 B1 proposes, in order to obtain maximum reflectivities, using very thin barrier layers of less than 0.35 nm thickness since these barrier layers generally have an absorbent effect at a wavelength around 13 nm.
Furthermore, US 2005/0270648 discloses mirrors for the EUV wavelength range around 13 nm for angles of incidence of up to 20°, which mirrors comprise so-called Mo/Si multiple coatings (Mo/Si multilayer).